DE68912994T2 - Prosthesis. - Google Patents

Description

This invention relates to a prosthesis for bone implantation, more specifically, for the femoral component of an artificial hip joint. A prosthesis having the characteristics of the preamble to claim 1 is already known from GB-A-2137098. Total hip endoprostheses have been in widespread use for over twenty years. One of the main problems relating to their use remains the long-term fixation of the prosthesis in the bone. The most commonly practiced fixation method is based on the use of a bone cement, such as polymethylmethacrylate, as a mortar between the metal prosthesis and the bone. However, there are certain disadvantages with this method, such as the fact that when the cement gives way, it tends to shatter. Small cement particles then cause an inflammatory reaction in the tissue, leading to bone resorption.

Over the last fifteen years, increasing interest has been directed toward fixation of the prosthesis without the use of cement. This requires an initial tight press fit of the prosthesis in the medullary canal. Various types of surfaces have been developed which provide rigid anchorage by locking the bone against the prosthesis. The current opinion is that bony apposition to the surface is likely to provide adequate long-term fixation even without a porous coating of the prosthesis surface, provided that the initial fixation is adequate.

Mammalian bones have two basic functions, first, to provide a lever-arm apparatus by which the animal can move, and second, to maintain the cells of the blood and immune system. These two roles are reflected in the two types of bone structure: hard, cortical bone, which bears the majority of the bone loads (in long bones it is thickest in the middle of the shaft and thinnest at the ends of the bone), and spongy medullary bone, which primarily provides a site for blood cell division. However, the trabeculae within the spongy bone can also be subjected to loads, and their contribution to load distribution is most significant at the ends of the bone, near the joints, where the cortex is thinnest.

It is known that bone responds to physical forces by laying down along the direction of loading, and bone subjected to physiological loading will grow in bone volume, and consequently in stability. Conversely, bone that is spared or not subjected to loading will lose bone volume and become weaker. All intramedullary femoral prostheses involve the removal of the trabeculae of cancellous bone in the femoral head and neck, so that all load transfer from the prosthesis must occur directly into the femur in the cortex. If the tip of the femoral prosthesis is clamped or wedged into the diaphysis, then load transfer will take place at that point, and the metaphyseal cortex will be spared the load. Consequently, it will tend to resorb. Ultimately, this will lead to failure of the implant fixation and require revision. The revision is made even more difficult technically by the poorer bone quality in the proximal metaphysis of the femur.

Theoretically, this problem can be solved if the proximal part of the prosthesis fits tightly and wedge-shaped into the metaphysis, while the distal prosthesis stem is inserted but not forced into the femoral diaphysis, where it acts merely as a keel, or "anti-tipping" device.

The relative sizes of the metaphyseal and diaphyseal marrow cavities vary not only from person to person but also with the age of the individual. With advancing age, the diaphyseal cortex is resorbed endosteally, resulting in a decrease in cortex and an increase in diaphyseal marrow cavities. Conversely, a corpulent, active man may have a very wide metaphyseal marrow cavity with very thick cortices in the diaphysis, resulting in a very narrow diaphyseal marrow cavity. This inherent variation becomes even more complicated when the combinations that can result from a previous implant failure are taken into account. Any combination of proximal or distal resorption can occur.

Prostheses are manufactured in a variety of sizes, but since the proximal and distal parts are usually enlarged or reduced together, they cannot accommodate the wide range of variations discussed above. Thus, numerous prostheses of different sizes must be provided, increasing the overall cost of the hospital's inventory.

EP-A-0257359 describes a modular hip prosthesis which is assembled from a kit containing a stem, a head and a neck. These individual components can be assembled in a modular manner and securely fastened together to form a custom-made prosthesis of a desired size and shape.

GB-A-2137098 describes an endoprosthetic bone joint device, particularly for use as part of a knee joint. The device consists of a loadable part which performs the articulation function at the bone end, a seat part which engages the growth zone of the bone and a usually tubular socket for pressure installation in the medullary canal of the bone. The loadable part is provided with a stem which fits slidably into the tubular socket so that the seat part and the loadable part can move outwards away from the tubular part as the bone grows, without affecting the fixation of the tubular part in the bone. A layer of resilient, shock-absorbing material can be placed between the loadable part and the seat part.

The present invention aims to create a prosthesis composed of components and an improvement in the fixation of the prosthesis in the bone, which reduces or completely prevents the resorption of the metaphyseal bone.

According to a first aspect of the invention there is provided a prosthesis for bone implantation comprising a proximal part which is inserted into the medullary canal of the bone metaphysis so as to protrude from the end of the bone and a distal part which is inserted into the medullary canal of the bone diaphysis, the proximal and distal parts being formed separately from one another and being adapted to fit together in such a way that they are capable of moving relative to one another in a direction which, in use, corresponds to the longitudinal direction of the bone, characterized in that the proximal part is shaped to fit into the metaphysis in a wedge shape and that the distal part is shaped to fit into the diaphysis so that most of the load exerted on the prosthesis is transferred through the proximal part to the metaphysis, whereby the two parts can be selected independently of each other to ensure an optimal fit in the bone, and the arrangement is such that the proximal part, after implantation, is able to move further into the bone towards the distal part while its position is consolidated without imparting an additional load to the distal part, so that the load transfer between the proximal part and the metaphysis is maintained

The invention provides a variety of proximal and distal parts of different sizes that can be interchangeably assembled to create a prosthesis as described above.

Preferred characteristics of this invention are apparent from the dependent claims, the technical data and the following description.

With the help of the accompanying drawings, the invention will now be further described, by way of example only:

Figure 1 shows the upper end of a femur;

Figures 2A, 2B and 2C show three different sizes of a prosthesis, according to an essence of the invention; and

Figure 3 shows one of the prostheses implanted into a femur.

The invention is described with reference to the femoral component of an artificial hip joint, but it will be understood that it is also applicable to prostheses of other types of joint. It is also applicable to prostheses for implantation in animals as well as in humans.

Figure 1 shows the components of the upper end of the femur as mentioned in the above discussion of the current method. The figure shows the head and neck of the femur, and illustrates the metaphyseal and diaphyseal bone parts. The cortex (cortical bone), the marrow (spongy bone) and the trabeculae are also shown. The endosteal surface between the medullary canal and the cortex is also seen.

Figure 2 illustrates an embodiment of a prosthesis according to the invention. Since, as discussed above, the proximal and distal parts have different functions to perform, they are supplied as separate parts. The prosthesis thus consists of a proximal part 1 shaped to fit into the medullary canal of the metaphysis and a distal part 2 shaped to fit into the medullary canal of the diaphysis. As described below, the proximal part 1 can be shaped to fit tightly into the metaphysis in a wedge-like manner, or a slightly smaller size can be selected which can be cemented in place. The distal part 2 is selected to fit into the diaphysis without being constrained or wedged, so that it acts as a keel or "anti-tipping" device.

The proximal and distal parts 1 and 2 are designed to fit together in such a way that different sized parts can be interchangeably mounted on one another to achieve the best possible fit in the bone. In detail, it is possible to mount a small proximal part 1, see Figure 2, onto a large distal part, see Figure 2C. Consequently, it is clear that, compared to the previous method in which the proximal and distal parts are completely formed, a large number of sizes and combinations can be created from relatively few components.

The proximal parts 1, shown in Figure 2, have a wedge-shaped piece 3, the tapered shape of which facilitates its anchoring in the metaphysis, and a neck 4 to which a head 5, such as a ball in the case of a ball-and-socket joint, can be attached, for example by means of a thread. By providing a selection of heads 5 with holes of different depths into which the neck 4 fits, the effective length of the neck 4 can be adjusted in a certain way. In some cases, the neck 4 is also available with a collar 6 which, in use, grips the outer surface of the bone (see Figure 3). The wedge-shaped piece 3 has an opening through which it can be attached to a distal part 2.

The distal part 2 (see Figure 2A) has an elongated shape, and consists of a parallel-sided shaft 8 which fits into the hole of the proximal part 1, and a tapered end piece 9 which fits into the medullary canal of the diaphysis. The larger distal parts 1, see Figures 2B and 2C, also have a parallel-sided middle piece 10 between the shaft 8 and the end piece 9, with a shoulder 11 between the shaft 8 and the middle piece 10 which has a larger diameter.

The holes 7 and the shafts 8 are all the same size (typically 12mm in diameter) so that different sized proximal and distal parts can be interchangeably mounted. The shaft 8 is preferably very tightly fitted into the hole 7, but as described below it is best to slide into it. It is also possible to imagine an arrangement (not shown) in which a stem attached to the proximal part fits into a hole in the distal part.

It will be understood that different sized proximal and distal parts can vary in a number of different dimensions. In the proximal part, for example, the radius R, the length L1 and the angle A can be varied. In the distal part, the lengths L2, L3 and L4 and the diameters D1 and D2 can vary. A typical prosthetic kit can consist, for example, of five different sized proximal parts and ten distal parts, with different lengths and/or diameters.

In addition to allowing different sized proximal and distal parts to be interchangeably assembled, another important feature of the prosthesis is that the proximal and distal parts 1 and 2, once assembled together, can move relative to each other in a direction parallel to the bone length. As indicated above, this is provided by a sliding fit between the shaft 8 of the distal part and the opening 7 of the proximal part.

This movement is desirable because the prosthesis and bone are known to move slightly relative to each other (especially in uncemented prostheses), especially during the first six months after implantation. By allowing the proximal part 1 to slide a few millimetres (e.g. up to 5mm) up and down the stem 8 of the distal part 2, it is able to bed, or settle, into the metaphysis, continually adjusting its compression fit in the metaphyseal bone, ensuring that the proximal load is optimally achieved. Since the proximal part 1 can slide on the stem 8 of the distal part 2, this movement is possible without the distal part being pushed further into the diaphysis, which could lead to wedging or jamming of the tip of the distal part in the diaphysis. The proximal part 1 can thus move a few millimetres until it is optimally pushed into the metaphysis, so that the load transfer between the proximal part and the proximal bone ensures that the latter does not suffer from resorption.

The shoulder 11 of the distal part 2 is arranged so that the proximal part 1 can slide further onto the stem 8 of the distal part 2 should the proximal part 1 move in the metaphysis when the prosthesis is loaded (see Figure 3). The shoulder 11 therefore does not prevent the embedding discussed above.

It will be understood that if the proximal and distal parts 1 and 2 were rigidly connected to one another (as in the fully supplied prostheses used to date), movement of the proximal part 1 in the metaphysis would likely push the distal part 2 further into the diaphysis and create a clamping fit between the distal part 2 and the diaphysis, which would shift the load transfer away from the proximal part 1 and bring about the problem of bone resorption discussed above.

Figure 3 shows a prosthesis implanted in a femur. As discussed above, the proximal and distal parts 1 and 2 are selected to provide the desired or optimal fit in the metaphysis and diaphysis of the bone. After removal of the head and neck of the bone, and removal of the cancellous bone from the metaphysis and diaphysis, the prosthesis is implanted in the bone so that the proximal part 1 fits in the metaphysis and the distal part 2 fits in the diaphysis without being trapped.

The proximal part 1 can be selected to be of the correct size to be wedged into the metaphysis without the use of cement. Consequently, the wedge-shaped piece 3 has a tapered shape corresponding to the shape of the medullary canal in the metaphysis when the cancellous bone is largely or has been removed altogether. Alternatively, particularly in elderly patients, it may be desired to cement the proximal part 1 into the metaphysis. In that case, select a slightly smaller proximal part 1 to allow space for the cement between the wedge-shaped piece 3 and the sides of the medullary canal in which it is to be fixed. In either case, select a distal part 2 which fits into the medullary canal of the diaphysis without being wedged or cemented therein, so that it simply acts as a clench or anti-tipping device and, for the reasons discussed above, bears little of the load exerted on the prosthesis.

If the proximal part is to be cemented into place, the two-piece structure of the prosthesis makes this easier because the distal part 2 can be implanted first before cement is applied to the bone. The proximal part 1 can then be cemented into the metaphysis and assembled to the distal part 2. The problem of cement seeping from the metaphysis into the medullary canal of the diaphysis, which can arise during the insertion of a one-piece prosthesis, is thus avoided.

It should be noted that even in cases where the proximal piece 1 has been cemented into the bone, there will still be some movement of the prosthesis upon initial loading. It is therefore still desirable to provide for movement between the proximal and distal parts 1 and 2.

Another advantage of the two-part structure of the prosthesis is its utility in the treatment of fractures and revisions. If a bone fractures around a loose-fitting prosthesis, for example through the diaphysis, it is difficult to insert a new prosthesis and hold the fracture together at the same time. However, with the prosthesis described above, the distal part 2 can be inserted into the diaphysis first to help hold the fracture together. It is much easier to insert the proximal part 1 into the metaphysis afterwards. Thus, instead of assembling the two parts of the prosthesis outside of the bone and then implanting them as a unit, the distal part 2 is first implanted alone and the proximal part 1 is then implanted into the metaphysis and mounted on the distal part 2.

The invention is applicable to prostheses made of a wide variety of materials, although the majority today are made of titanium or an alloy thereof. The various surface finishes used in this field can also be applied to the prosthesis and, given the two-part structure, it is easy to assemble the proximal and distal parts 1 and 2 with different surface finishes, should this be desired.

It will also be understood that the design of a variety of known prostheses can be readily modified to provide them with the two-part structure described above and to allow the two parts to move relative to each other as described.

One can also imagine other methods for assembling the two parts, as well as alternative methods that allow the movement in question.

Claims (7)

1. A prosthesis for bone implantation, comprising a proximal part (1) which is inserted into the medullary canal of the bone metaphysis so as to protrude from the bone end, and a distal part (2) which is inserted into the medullary canal of the bone diaphysis, the proximal and distal parts (1, 2) being formed separately from one another and adapted to fit together in such a way that they are able to move relative to one another in a direction which, in use, corresponds to the longitudinal direction of the bone, characterized in that the proximal part (1) is shaped to fit into the metaphysis in a wedge shape and that the distal part (2) is shaped to fit into the diaphysis so that most of the load exerted on the prosthesis is transferred to the metaphysis through the proximal part, whereby the two parts (1, 2) can be selected independently of one another, to ensure an optimal fit in the bone, and wherein the arrangement is such that the proximal part (1) after implantation is able to move further into the bone towards the distal part (2) while its position is fixed without imparting an additional load to the distal part, so that the load transfer between the proximal part (1) and the metaphysis is maintained. 2. A prosthesis according to claim 1, in which the proximal part (1) has a neck (4) and a device by means of which a head part (5), e.g. the ball of a ball joint, can be fastened to it. 3. A prosthesis according to claim 1 or 2, in which a movement of up to 5 mm between the proximal and distal parts (1, 2) in said direction is possible. 4. A prosthesis according to any preceding claim, wherein the proximal and distal parts (1, 2) are adapted to be assembled by means of a shaft (8) formed on one part and fitting into a hole (7) in the other part. 5. A prosthesis according to claim 4, in which the shaft (8) is attached to the distal part (2) and the hole in the proximal part (1). 6. A prosthesis according to any preceding claim, comprising the femoral component of an artificial hip joint. 7. A plurality of proximal and distal parts of various sizes which can be interchangeably assembled to form a bone implantation prosthesis comprising a proximal part (1) which is inserted into the medullary canal of the bone metaphysis so as to protrude from the end of the bone and a distal part (2) which is inserted into the medullary canal of the bone diaphysis, the proximal and distal parts (1, 2) being formed separately from one another and adapted to fit together in such a way that they are able to move relative to one another in a direction which, in use, corresponds to the longitudinal direction of the bone, the proximal part (1) being shaped to fit in the metaphysis in a wedge shape and the distal part (2) being shaped to fit in the diaphysis so that most of the load exerted on the prosthesis is borne by the proximal part is transferred to the metaphysis, whereby the two parts (1, 2) can be selected independently of each other to ensure an optimal fit in the bone, and wherein the arrangement is such that the proximal part (1) after implantation is able to move further into the bone towards the distal part (2) while its position is consolidated without transferring an additional load to the distal part, so that the load transfer between the proximal part (1) and the metaphysis is maintained.